insert_many ordered param should default to false not true

[amorton]

https://github.com/datastax/astrapy/blob/9f039ad25e3628b840ad67e516e86d433150351d/astrapy/collection.py#L609C1-L610C1

The default in the API is false because it is much much faster, and the documentation for the API says users should set it top false. We should change the default to be false to give the best performance.